This application claims priority of European Patent Application No. 98306720.8, which was filed on Aug. 21, 1998.
1. Field of the Invention
The invention relates to a CDMA system, including a transmitter and receiver, for use in e.g. a digital wireless communications system. In particular, the invention relates to a method of and apparatus for mapping and demapping CDMA signals.
2. Description of Related Art
Recently iterative decoding algorithms have become a vital field of research in digital communications. The first discovered and still most popular encoding scheme suited for iterative decoding is the parallel concatenation of two recursive systematic convolutional codes, also referred to as xe2x80x98Turbo Codesxe2x80x99, as described in J. Hagenauer, xe2x80x9cThe Turbo Principle: Tutorial Introduction and State of the Artxe2x80x9d, Symposium on Turbo Codes, Brest, France, September 1997. In the past few years other applications of the xe2x80x98Turbo Principlexe2x80x99 have been found e.g. G. Bauch, H. Khorram, J. Hagenauer, xe2x80x9cIterative Equalization and Decoding in Mobile Communications Systemsxe2x80x9d, in Proc. EPMCC""97, Bonn, pp. 307-312, Germany, October 1997.
Channel coding is used to make the transmitted digital information signal more robust against noise. For this the information bit sequence is encoded at the transmitter by a channel encoder and decoded at the receiver by a channel decoder. In the encoder, redundant information is added to the information bit sequence in order to facilitate error correction in the decoder. For example, in a systematic channel encoding scheme the redundant information is added to the information bit sequence just as additionally inserted, xe2x80x98codedxe2x80x99 bits. In a non-systematic encoding scheme the outgoing bits are all coded bits, and no xe2x80x98nakedxe2x80x99 information bits remain. The number of incoming bits (information bits) at the encoder is smaller than the number of outgoing bits (information bits plus inserted coded bits, or all coded bits). The ratio of incoming/outgoing bits is called the xe2x80x98code rate Rxe2x80x99 (typically R=xc2xd).
Concatenated coding schemes apply to at least two parallels or serially concatenated encoders. There are iterative decoding algorithms for either parallel or serially concatenated coding systems and the classical xe2x80x98Turboxe2x80x99 Codes are parallel concatenated codes.
FIG. 1 shows a genuine serially concatenated coding scheme with the transmission performed on a block-by-block basis. The signal sequence is encoded twice at the transmitter in a serial manner. The binary signal from the digital source (e.g. an analogue to digital converter with analogue input signal from a microphone) is first encoded by an outer encoder. The output of the outer encoder gets passed through an interleaver which changes the order of the incoming bit symbols to make the signal appear more random to the following processing stages. After the interleaver the signal is encoded a second time by an xe2x80x98inner encoderxe2x80x99. Correspondingly, at the receiver the signal is first decoded by the inner decoder, deinterleaved, and decoded by the outer decoder. From the outer decoder soft values are fed back as additional xe2x80x98a priorixe2x80x99 input to the inner decoder. The soft values are reliability values of the quality of the decoded signal. The feedback of these values helps to reduce the bit error rate of the hard decision values 0,1 at the output of the outer decoder in further, iterative decoding steps. The iterative decoding of a particular transmitted sequence is stopped with an arbitrary termination criterion, e.g. after a fixed number of iterations, or until a certain bit error rate is reached (the termination criterion is not important at all for the invention). It should be noted that the xe2x80x98a priorixe2x80x99 soft value input to the inner decoder is set to zero for the very first decoding of the transmitted bit sequence (xe2x80x980th iterationxe2x80x99).
The inner and outer binary codes can be of any type: Systematic, or non-systematic, block or convolutional codes.
At the receiver the two decoders are soft-in/soft-out decoders (SISO decoder). A soft value represents the reliability of the bit decision of the respective bit symbol (whether 0 or 1). A soft-in decoder accepts soft reliability values for the incoming bit symbols. A soft-out decoder provides soft reliability output values on the outgoing bit symbols. The soft-out reliability values are usually more accurate than the soft-in reliability values since they can be improved during the decoding process based on the redundant information that is added with each encoding step at the transmitter.
Method and apparatus for iteratively demapping a signal is described in European Patent Application number 98302653.5 filed on Apr. 3, 1998, the contents of which are incorporated herein by reference.
CDMA (Code Division Multiple Access) systems have a plurality of users separated by different codes. Each transmitted bit of a user k is substituted by a number Nc of xe2x80x98shorterxe2x80x99 bits, called xe2x80x98chipsxe2x80x99, which are chosen according to the xe2x80x98channelization codexe2x80x99 of the particular user k. Since the occupied bandwidth expands by a factor of Nc after the xe2x80x98substitutionxe2x80x99, this process is called xe2x80x98spreadingxe2x80x99, and a CDMA system is often referred to as xe2x80x98spread spectrum systemxe2x80x99. After spreading, the signal for each user occupies the total available bandwidth B. At the receiver the desired user is detected by means of correlation with the appropriate channelization code.
Multicode CDMA is a method that provides higher data rates to a single user in a CDMA system. The particular user is assigned N channelization codes, instead of having only one in a conventional CDMA system. Hence the xe2x80x98multicodexe2x80x99 user can transmit at an N times higher data rate than a xe2x80x98single-codexe2x80x99 user. The N binary antipodal codes are added up at the multicode transmitter to form a N+1-level amplitude modulated signal, instead of a binary antipodal signal for the single code case.
FIG. 2 shows a conventional multicode CDMA (M-CDMA) single user transmitter/receiver. The binary signal from the digital source (e.g. an analogue to digital converter with analogue input signal from a microphone) is first encoded by the channel encoder. The output of the channel encoder gets passed through an interleaver which changes the order of the incoming bit symbols to make the signal appear more random to the following processing stages. After the interleaver the coded bit stream is split into N parallel bit streams by a demux (serial to parallel conversion). Each bit stream 1, . . . ,N gets spread (i.e. multiplied) by a binary antipodal channelization code (codeword 1, . . . ,N) of length Nc chips. Typically, the N channelization codes are orthogonal. After spreading the N binary antipodal chip streams are added up on the chip rate to form the amplitude modulated (N+1 levels) chip symbols. Hence N coded bit symbols result in Nc chip symbols. Each block of Nc chip symbols is referred to as a composite multicode CDMA symbol. Optionally, scrambling on the chip rate can be applied to even further randomize the signal. Typically the scrambling sequence is a binary antipodal pseudo-random sequence. The composite signal is then put to the transmission channel.
FIG. 2 shows base band processing only and up-conversion to radio frequency etc. is omitted for the sake of clarity. For simplicity of the description we assume a real signal processing. However, the real channelization codes could be complex channelization codes as well, or there could be a complex scrambling sequence.
On the channel the signal is distorted by additive noise, or any other noise form.
Correspondingly, at the receiver the signal is descrambled (optionally) and correlated with the N channelization codes (codewords 1, . . . ,N). The correlation consists of a multiplication with the respective channelization code and an accumulation over Nc chips. After multiplexing (parallel to serial conversion) and deinterleaving the N correlation results are put to the channel decoder. Finally, the information bits are available at the output of the decoder, or the hard decision device respectively.
The conventional CDMA and M-CDMA system as described above does not allow for bit error rate (BER) reduction e.g. though iterative decoding. There is thus a requirement for a CDMA or M-CDMA system in which an improved BER may be achieved.
According to a first aspect of the invention there is provided a CDMA system comprising: a transmitter including an encoder and a bit interleaver for processing said CDMA signal to be transmitted; means for transmitting said processed CDMA signal; means for receiving said transmitted CDMA signal; a receiver including a bit deinterleaver and a decoder for processing said received CDMA signal; and characterized in that the transmitter includes a mapper connected in series with the encoder and bit interleaver and the receiver includes a demapper connected in series with the bit deinterleaver and decoder.
According to a second aspect of the invention there is provided a CDMA transmitter comprising: an encoder and a bit interleaver for processing said CDMA signal to be transmitted; means for transmitting said processed CDMA signal; characterized in that the transmitter includes a mapper connected in series with the encoder and bit interleaver.
According to a third aspect of the invention there is provided a CDMA receiver comprising: means for receiving said transmitted CDMA signal; a bit deinterleaver and a decoder for processing said received CDMA signal; and characterized in that the receiver includes a demapper connected in series with the bit deinterleaver and decoder.
In the receiver iterative demapping is performed by passing back the output of the decoder to the demapper. The system may be a multi-user CDMA system, in which case the N parallel channelization codes are regarded as the channelization codes of at most N different users. The system may be a multicode CDMA system in which case the N channelization codes belong to at least one user.
According to a fourth aspect of the invention there is provided a method of transmitting a CDMA signal comprising the steps of: generating a CDMA signal to be transmitted; encoding and bit interleaving said CDMA signal; transmitting said encoded and interleaved CDMA signal; and characterized in that the encoded and interleaved CDMA signal is mapped before being transmitted.
There is also provided a method of receiving a CDMA signal transmitted as described above comprising receiving the transmitted CDMA signal; bit deinterleaving and decoding the received CDMA signal; and characterized in that the received CDMA signal is demapped before being deinterleaved and decoded.
The CDMA signal is iteratively demapped by iterating the bit deinterleaving, decoding and demapping steps.
By inserting a mapper between demultiplexer and orthogonal spreading of the N parallel CDMA channels of a conventional transmitter, a bit error, rate reduction through iterative decoding can be achieved at the receiver. For this, the receiver has to perform a demapping operation after the despreading. The demapper can make use of a priori knowledge gained by the channel decoder that enables iterative demapping and decoding.
The xe2x80x98modifiedxe2x80x99 mapping can be arbitrary and just needs to be different from the xe2x80x98identicalxe2x80x99 mapping. However, the achievable performance gains strongly depend on the chosen xe2x80x98modifiedxe2x80x99 mapping. The mapper does not add redundancy to the signal and can be interpreted as a rate one binary block encoder in combination with orthogonal CDMA spreading. However, the spreading does not have to be orthogonal.
Iterative demapping and decoding reduces the bit error rate of multicode CDMA links that use plain channel coding.
Iterative demapping and decoding for CDMA works as long as there is at least one bit symbol interleaver between encoder and the spreading/mapping.
Since the orthogonality of the spreading codes is preserved, this method is suited, e.g., for synchronous multi-user down-link communication in a wireless communication link (base station to mobile terminal, multi-user transmitter), as well as for asynchronous up-link communication (mobile terminal to base station, single user transmitter).